Why not just forward emails to people who need to see them?
Because when you forward separate emails, or long threads, the recipient has to try to piece it all together by deciphering a bunch of deeply indented quoted emails, reading backwards in time from the bottom right to the top left. It sucks. But with a Collection, you just grant them instant access to everything in one fell swoop. They see what you see, and everything’s organized together on one screen. It’s like magic.
Why would I use a Collection rather than a Label?
Labels are like tags — they’re great for loosely organizing separate threads in the same general category. Think receipts from “2021 Charitable Contributions”. The receipts don’t have anything to do with one another other than being a charitable contribution in 2021. Collections, however, are perfect for organizing multiple threads about the same topic. Like a project. Maybe you’re renovating your office, and you’re juggling 7 or 8 email threads about that renovation project. Making an “Office Renovation” Collection, and adding those threads to that collection, will allow you to follow all the different conversations about that project in a single place. And if you want someone else to see all the emails in that Collection, you can just share the Office Renovation Collection with them, and they’ll have instant, full access to every email thread that’s part of that Collection.
Why not merge a bunch of emails together rather than creating a Collection?
You could, but then you’d end up with a single megathread which may be hard to follow over time. It’s often nice to keep separate threads separate, but keep them organized together in a Collection instead.